In agricultural vehicles, it is not practical to add engine oil for lubrication by pouring it into a valve cover at the top of the engine. For this reason, it is known to use the access hole for the dipstick as the filler hole for adding oil to the engine. When the dipstick access hole is made larger to permit oil to be poured into the engine through it, there is an increased tendency for the dipstick to be ejected by the crankcase pressure acting on the cap closing off the filler hole.
An oil filler tube assembly for an internal combustion engine that mitigates this problem is disclosed in U.S. Pat. No. 5,022,495. This assembly includes a tube having a lower end communicating with the crankcase of the engine and an open upper end that is closed by a cap. The under-surface of the cap carries a dipstick which extends downwardly within the tube. The upper end of the tube has a pair of locking elements which are spaced circumferentially with the spaces between the elements being of different circumferential dimension. The cap of the dipstick is constructed in a manner that resembles a conventional engine radiator cap, the cap having a peripheral flange of which the lower edge has a pair of locking lugs of different circumferential dimension. The locking lugs on the cap are constructed and arranged to be received within the spaces between the locking elements of the tube as the cap is inserted on the tube. The locking elements and the locking lugs are provided with mating cam surfaces so that when the cap is rotated one quarter of a turn, a seal on the undersurface of the cap will be forced against the upper end of the tube to tightly seal the cap to the tube.